1. Field of the Invention
This invention relates to methods and apparatus for generating an AC output waveform of one frequency from a multiphase AC input waveform of another frequency and more particularly for controlling the input power factor in such a power conversion.
2. Description of the Prior Art
Solid state frequency changers synthesize an output waveform of one frequency from a multiphase input waveform of another frequency by sequentially switching segments of each phase of the input waveform to the output. In some such frequency changers an output waveform of constant frequency is generated from an input waveform of variable frequency. These units are referred to as variable-speed-constant-frequency (VSCF) power converters. A practical application of such a frequency changer is an aircraft electrical system wherein fixed frequency power is derived from an AC generator driven at variable speed by the aircraft engine.
My copending application referred to above discloses a VSCF power converter which is operated as an unrestricted frequency changer (UFC) at low generator frequencies and is operated in a complementary mode at high frequencies. Under UFC operation, the frequency changer sequentially switches to input phases which are more positive when the fundamental component of the output waveform is positive going and to input phases which are more negative when the output fundamental is negative going. The switching pattern is reversed under complementary UFC (CUFC) operation; that is, switching is to input phases which are more negative when the output waveform fundamental component is positive going and to input phases which are more positive when it is negative going. Since the switching frequency is lower in the complementary mode of operation which is used at the higher generator frequencies, the switching frequency over the entire range of generator frequencies does not vary widely. This increases the efficiency of the frequency changer and permits the use of slower solid state switching devices than would otherwise be required.
Another feature of the frequency changer disclosed in my copending application is that under UFC operation the frequency changer inverts the load phase angle. Thus, since the load in aircraft electric power systems is normally inductive producing a lagging load power factor, the generator sees a leading power factor. This leading input power factor aids in the excitation of the generator and is the reason that UFC operation is used at low generator frequencies. On the other hand, the CUFC mode of operation results in an input power factor that is the same as the load power factor. This lagging input power factor reduces the generator terminal voltage at high generator frequencies where this mode of operation is used.
While the phase angle of input power factor of the frequency changer of my copending application can be inverted, the magnitude of the phase angle is dictated by the load power factor.
It is the primary object of the present invention to provide an AC to AC power converter in which the magnitude as well as the character of the input power factor can be controlled. It is also an object of the invention to provide a novel means for controlling the voltage of a generator driven at variable rpm.